Xylitol is a low-calorie sweetener, which is increasingly used in food, pharmaceutical and oral health products. It is a naturally occurring five-carbon sugar alcohol (polyol). Emil Fischer, a German chemist, discovered it in 1891. It is used as a sweetening agent in human food since the 1960s and is a white crystalline powder, odorless and has a pleasant taste. Xylitol is found in many fruits and vegetables and also produced by human body during normal metabolism. It is the sweetest of all polyols. It is as sweet as sucrose and has no after taste and is suitable for diabetics. It has 40% less calories than sucrose (EU, US) and has a calorific value of 2.4 kcal/gm. It is the only sweetener with conclusive data on caries prevention and has strong cariostatic potential. It has also gained importance in the prevention of acute otitis media (most common ear infection in children).
Xylitol can be produced by either chemical or biological methods, usually being produced by the chemical reduction of xylose derived from hemicellulosic fractions of birchwood or other xylose-rich materials. Xylitol is produced commercially by extraction from birch wood or other hemicellulosic material such as bagasse, which is rich in xylans. The fermentation of xylan rich material to xylose has been reported in literature. Yeasts generally produce Xylitol. The biosynthetic pathway involves isomerisation of D-Xylose to D-xylulose by xylose isomerase and reduction to xylitol by NADH-dependent xylitol dehydrogenase or reduction by xylose reductase in the presence of NADPH or NADH.
Reference may be made Yong. K. S. et al in “Fermentation process for preparing xylitol using Candida tropical is in U.S. Pat. No. 5,998,181, 1999 wherein the authors have reported a xylitol yield of 19.6% based on D-xylulose from Pichia farinosa ATCC 20216) and a yield of 48% from Zygosaccharomyces polymorphus. 
Reference may be made to Yong. K. S. et al. in Fermentation process for preparing xylitol using mutant cells” in U.S. Pat. No. 5,686,277, 1997 wherein the authors have reported a xylitol yield of 80-85% based on xylose from Candida parapsilosis (KCCM 10088).
Reference may be made to Heikki. O. et al in ‘Process for the preparation of xylitol from xylose by cultivating Candida guilliermondif in WO patent No. 8805467, 1988 wherein the authors have reported a xylitol yield of 63-78% based on xylose from Candida guilliermondii (VTT-C-71QQ6) by fed-batch fermentation.
Reference may be made to Rahkila, L. et al., in “Method for the production of xylitol” in U.S. Pat. No. 5,081,026, 1992 wherein the authors have reported a xylitol yield of 50% based on xylose from Debaryomyces hansenii. 
Reference may be made to Walther, T et al. [Bioresource-Technol. 76:3, 213-220. 2001] wherein the authors have reported a maximum xylitol yield of 84% from Andida tropicalis based on xylose.
Reference may be made to Choi, H. et al [Biotechnol. Lett. 22:20. 1625-1628, 2000] wherein the authors have reported a maximum xylitol yield of 82% from Candida tropicalis (ATCC 3803) based on xylose.
Reference may be made to Bao. X. et al [Ind. Microbiol. 30:2, 3-18, 2000] wherein the authors have reported a maximum xylitol Yield of 94% from a recombinant strain of Saccharomyces cerevisiae (strain HYEX2) using recombinant xylose reductase gene.
Reference may be made to Tavares. J. M. et al. [Enzyme Microb. Technol. 26:9-10. 743-747. 2000] wherein the authors have reported a maximum xylitol yield of 56% based on xylose from Debaryomyces hansenii (CCMI 941).
Reference may be made to Nakano, K. et al. [J.Ferment.Bioeng. 89:4, 372-376, 2000], wherein the authors have reported a maximum xylitol yield of 82% based on xylose from Candida magnoliae TISTR5663.
Reference may be made to Suryadi. H et al., [J.Ferment.Bioeng. 89:3, 236-240, 2000] wherein the authors have reported a maximum xylitol yield of 46% based on xylose from Hansenula polymorpha DL1 (AKU 4327).
Reference may be made to Converti, A. et al., [Appl.Biochem.Biotechnol. 82:2, 141-151. 2000] wherein the authors have reported a maximum xylitol yield of 63% based on xylose from Pachysolen tannophilus (NRRL Y-2460).
Reference may be made to Preziosi, B. L. et al., [Biotechnol.Lett. 22:3, 239-243, 2000] wherein the authors have reported a maximum xylitol yield of 80% based on xylose from Candida guilliermondii (NRC 5578).
Reference may be made to Rodrigues, D. C. G. A et al., [Lett.Appl.Microbiol. 29:6, 359-363. 1999] wherein the authors have reported a maximum xylitol yield of 84% based on xylose from Candida guilliermondii (FTI 20037).
Reference may be made to U.S. Patent. No. US 2002164731 entitled, “Process for the simultaneous production of xylitol and ethanol” indicates a fermentation time of 24 to 96 hrs, however, the yields of xylitol was poor.
Reference may be made to Parajo, et.al. [Bioresource Technol. 65, 203-212, 1998] wherein the review paper shows the fermentation time ranges from 24-800 hours, depending on the culture used for xylitol production. The cultures producing xylitol in a fermentation time shorter than 72 hours have low xylitol yields based on xylose, with the only exception of a mixed culture of C. pelliculosa and Methanobacterium sp. where the yields were 1 g of xylitol g−1 of xylose. The draw back of the above said process was maintenance of consortium of bacterial strains and higher operating cost.
The drawback of the above references cried by various authors is relatively low yields of xylitol. Since, the final concentration of xylitol depends on the amount of xylose used, the xylitol yield from xylose represents the efficiency of the culture to produce xylitol. A summary of the fermentative yield of xylitol from xylose for different strains is shown in Table 1.